1. Field of the Invention
The present invention relates to an image recording apparatus and method, and a method of specifying density correction coefficients, and more particularly to image processing technology which is suitable for correcting density variations caused by variation in characteristics between recording elements of a recording head having a plurality of recording elements.
2. Description of the Related Art
In an image recording apparatus (inkjet printer) having an inkjet type of recording head comprising a plurality of ink ejection ports (nozzles), problems of image quality arise due to the occurrence of density variations (density non-uniformities) in the recorded image caused by variations in the ejection characteristics of the nozzles. FIG. 15 is an illustrative diagram showing a schematic view of examples of variations in the ejection characteristics of the nozzles, and density variations appearing the recording result.
In FIG. 15, a line head 300 has nozzles 302-i (where i=1 to 8), which eject droplets of ink toward a recording medium (for example, recording paper) to form dots 304-i (i=1 to 8), respectively, on the recording medium. The recording medium is moved in the direction of an arrow S (the sub-scanning direction) relatively with respect to the line head 300.
In the example shown in FIG. 15, a depositing position error occurs at the nozzle 302-3, which is third from the left (namely, the droplet ejected from the nozzle 302-3 lands on the recording medium at a position diverging from the originally intended depositing position in the leftward direction in FIG. 15), and a droplet volume error occurs at the sixth nozzle 302-6 (namely, the droplet ejected from the nozzle 302-6 has a greater droplet volume than the originally intended volume). In this case, density non-uniformity streaks occur at the positions in the print image corresponding to the nozzles 302-3 and 302-6 producing the depositing position error and the droplet volume error (namely, the positions indicated by A and B in FIG. 15).
In the case of a serial or shuttle-scanning type of image recording apparatus, which performs image recording by driving a recording head to scan a plurality of times over the prescribed print region, it is possible to avoid density non-uniformities by means of a commonly known multi-pass printing method, but in the case of a line head system (full width array) which records images by means of a single scanning action, it is difficult to avoid density non-uniformities.
Since it is difficult to completely prevent variations in ejection characteristics between the nozzles in manufacturing terms, then various technologies for correcting the variations have been proposed (see, Japanese Patent Application Publication Nos. 04-018363 and 2004-058282). As shown in FIG. 15, the principal causes of density non-uniformities are variation in the droplet volume and variation in the depositing position. Japanese Patent Application Publication No. 04-018363 chiefly discloses technology for correcting density non-uniformities caused by error in the droplet volume. Japanese Patent Application Publication No. 2004-058282 chiefly discloses technology for correcting density non-uniformities caused by error in the depositing position.
The principles of correction methods in the related art are now described generally with reference to FIG. 16. In FIG. 16, the third nozzle from the left (NZ3) has a depositing position error (namely, characteristics whereby the droplet ejected from the nozzle NZ3 lands on the recording medium at a position diverging from the originally intended depositing position, in the rightward direction in the diagram). The graph shown in the bottom part of FIG. 16 indicates the density profile in the nozzle column direction (main scanning direction), in which the print density produced by the droplets ejected from the nozzles is averaged per nozzle in the conveyance direction of the recording medium (the sub-scanning direction). The horizontal axis (X axis) represents the position in the main scanning direction, and the vertical axis represents the optical density (O.D.).
In general terms, the correction principle described in Japanese Patent Application Publication No. 04-018363 is as described below.
Step 1: Firstly, the densities of areas (density measurement areas AR1 to AR5) corresponding to the ideal positions of nozzles NZ1 to NZ5 are measured (or they are calculated arithmetically from a prescribed model).
Step 2: The nozzle output values are specified on the basis of the area densities thus measured (or calculated), in such a manner that the area densities are made uniform.
In the case of FIG. 16, the density of the area AR3 is reduced in comparison with ideal droplet ejection (as indicated by the dashed line), whereas the density of the area AR4 is increased. Therefore, processing (output value correction) is carried out in order to raise the output value of the nozzle NZ3 and reduce the output value of the nozzle NZ4, in qualitative terms. This is equivalent to controlling the height of the peak in each density measurement area, but this does not necessarily require control of the ejection volume of each nozzle, but rather, the output value can be controlled on an average basis, by modifying the number of dots in the sub-scanning direction printed by each nozzle. The number of dots can be adjusted by controlling the image processing, and therefore it allows the density to be corrected in a simple fashion.
Japanese Patent Application Publication No. 2004-058282 discloses correction processing which can be regarded as an improvement of the correction processing disclosed in Japanese Patent Application Publication No. 04-018363. To give a general description of the correction processing according to Japanese Patent Application Publication No. 2004-058282, (1) firstly, depositing position error information for each nozzle is acquired by means of a special test pattern; (2) the density characteristics of the print area corresponding to a particular nozzle are inferred by taking account of the effects of depositing position errors in the adjacent nozzles; and (3) output correction is carried out on the basis of the inferred density characteristics.
As shown in FIG. 17, weighting relationship Z(NZ→AR) between the nozzle output and the area density is designated, and a nozzle control amount is specified on the basis of this weighting relationship Z, in such a manner that the area densities become uniform. FIG. 17 shows an example of weighting of the nozzle output, and the weighting relationship is specified by taking account of the surface area occupied by the dots, and the dot density profile (which is generally an approximate hemispherical shape as shown in FIG. 17).
In the case of the nozzle NZ3 shown in FIG. 16, for example, taking account of the effects (density contribution) of the dot density profile (solid line) created due to the depositing position error, on the areas AR2, AR3 and AR4 as shown in FIG. 17, the values of the weighting relationship Z are Z(3→2)=0.0, Z(3→3)=0.8, and Z(3→4)=0.2. By using such weighting relationship Z, the inconsistency between the nozzle position and the area position is removed, and control can be implemented more precisely in order to make the area densities uniform.